Thanks, this answers a lot. Reading a bit further, it's interesting that everyone focuses on the legality of the government requesting and receiving information

Thanks, this answers a lot. Reading a bit further, it's interesting that everyone focuses on the legality of the government requesting and receiving information rather than on the legality of the company giving it to them. Can a company that hands over information "without a fight" be sued for a breach of some implicit trust? If not, what about explicit promises or contracts? If at some point the company is not allowed to freely hand over the information, is the government still allowed to receive it in breach of the confidentiality arrangement? And what about companies giving this information to other non-governmental third parties? In a lot of the contracts I've read and signed, companies have explicitly informed me of circumstances in which (and people to whom) my information may be disclosed. What does this imply about people not named, and what is implied by a contract not having such a clause?